Ball valves cooperating with conical valve seats are used, for example, with fuel-injection valves for internal-combustion engines and the valve assembly formed by the valve ball and the seat can be provided in very small dimensions with the conical seat being formed in a machine operation and by grinding. As a rule, in the past, the conical seat has been so made that a line contact is formed between the conical surface and the grinding tool.
When conical seats are made in this fashion, it is found that in transverse cross section planes, i.e. cross sections in planes perpendicular to the axis of the conical seat or ground surface, there are deviations of that surface from a perfect circle. These deviations are referred to herein as shape deviations and when the valve assembly is used as a fuel-injection valve of the type described, such shape deviations can exceed 1 .mu.m from the mean diameter and can be sufficient to give rise to leakage even when the valve is intended to be closed. It will be understood that a mean diameter of such a conical valve seat can be several millimeters. The leakage rates which result have been found to be detrimental to the operation of the internal-combustion engine.